Heat is generated within internal combustion engines, principally from the compression and combustion of the fuel and air mixture and, additionally, from the friction of engine parts moving relative to one another, most notably the piston rings and cylinder walls. In conventional automotive engines, the engine heat is removed by transfer to a coolant liquid flowing through a water or coolant jacket of the engine and from the coolant to the ambient atmosphere in the "radiator" or coolant-to-air heat exchanger.
As the engine operating time increases, frictional wear between mutually moving parts loosens the fit between such parts, such that the proportion of heat generated by friction decreases. At the same time that engine wear is occurring, the cooling efficiency of the radiator and cooling system is decreasing due to corrosion, the deposit of contaminants within the coolant onto the internal surfaces of the various passages of the cooling system, and other factors. Thus, the loss in cooling efficiency is normally balanced by a decrease in cooling requirements from the reduction in frictionally generated heat.
Although the great majority of automobile engines are discarded when worn out, along with the vehicles in which they are installed, worn engines are sometimes rebuilt if the vehicle body is otherwise viable. Also, if an engine fails early in the life cycle of a vehicle, it is often more economical to replace the engine than to buy a replacement vehicle. Such replacements are often rebuilt engines, although new engines are also used as replacements.
When engines are rebuilt, components which have been worn by friction are replaced or remachined, as appropriate. This usually involves resurfacing of the cylinder walls and the installation of oversized pistons and rings, along with the replacement of bearings and bearing structures. A new engine or a rebuilt engine is frictionally "tighter" than a worn engine such that frictionally generated heat is also greater. It is generally recommended that the cooling system of a relatively older vehicle be replaced or carefully checked when a rebuilt or new engine is installed because of the increased cooling requirements of a tighter engine. If a new or rebuilt engine is installed in a vehicle with an inadequate cooling system, engine failure resulting from overheating can occur. It should be noted that rebuilt engine failures can result from other factors which may be attributable to the engine rebuilder, the rebuilt engine installer, or to the vehicle operator.
In order to protect themselves from unfair warranty claims related to such engine failures, some engine rebuilders install heat indicator tabs at various locations on the block and heads of rebuilt engines to indicate if the engine has experienced overheating. One well known type of heat indicator tab is a small disk formed by an outer ring frame surrounding a center disk of a metal that melts at a temperature which would indicate overheating of the engine. The fusible disk may, for example, be an alloy of bismuth and lead in equal proportions, which melts at about 250.degree. F. The tab assembly is installed on a cleaned engine surface using a high temperature tolerant adhesive. Ideally, contact between the engine surface and the fusible disk allows engine heat to be transferred to the fusible disk. If the engine reaches an operating temperature which exceeds the melting temperature of the fusible disk, it melts and drains away from the heat indicator tab, giving visible evidence of overheating.
One functional problem with such conventional heat indicator tabs is that if they are not installed carefully, a layer of adhesive can be left between the heat indicator tab and the engine surface which can form a heat insulator therebetween. Another possibility is that an air pocket can be left between the fusible disk and the surface of the engine to be monitored which can also act as an insulator therebetween. In either case, the engine may overheat and fail before the fusible disk reaches its melting point.